OSLO, Sept 10 (Reuters) - The world needs to find theequivalent of the flow of 20 Nile rivers by 2025 to grow enoughfood to feed a rising population and help avoid conflicts overwater scarcity, a group of former leaders said on Monday.

Factors such as climate change would strain freshwatersupplies and nations including China and India were likely toface shortages within two decades, they said, calling on theU.N. Security Council to get more involved.

"The future political impact of water scarcity may bedevastating," former Canadian Prime Minister Jean Chretien saidof a study issued by a group of 40 former leaders he co-chairsincluding former U.S. President Bill Clinton and Nelson Mandela.

"It will lead to some conflicts," Chretien told reporters ona telephone conference call, highlighting tensions such as inthe Middle East over the Jordan River.

The study, by the InterAction Council of former leaders,said the U.N. Security Council should make water the topconcern. Until now, the Security Council has treated water as afactor in other crises, such as Sudan or the impact of globalwarming.

It said that about 3,800 cubic km (910 cubic miles) of freshwater was taken from rivers and lakes every year.

"With about 1 billion more mouths to feed worldwide by 2025,global agriculture alone will require another 1,000 cubic km(240 cubic miles) of water per year," it said. The worldpopulation now is just over 7 billion.

The increase was "equal to the annual flow of 20 Niles or100 Colorado Rivers", according to the report, also backed bythe U.N. University's Institute for Water, Environment andHealth (UNWEH) and Canada's Gordon Foundation.

CHINA, INDIA

It said the greatest growth in demand for water would be inChina, the United States and India due to population growth,increasing irrigation and economic growth.

"By 2030, demand for water in India and China, the mostpopulous nations on Earth, will exceed their current supplies,"the report said.

Global warming, blamed on human emissions of greenhousegases from burning fossil fuels, would aggravate the problems.

"We say in the U.N. system that climate change is all aboutwater," said Zafar Adeel, director of UNWEH. Severe weatherevents - such as droughts, floods, mudslides or downpours - werebecoming more frequent.

UN-Water, which coordinates water-related efforts by theUnited Nations, will organise a meeting of foreign ministersthis month and separate talks among experts on Sept. 25 to lookat ways to address concerns over water.

The report said there were examples of water-relatedconflicts, for instance between Israelis and Palestinians overaquifers, between Egypt and other nations sharing the Nile, orbetween Iran and Afghanistan over the Hirmand River.

But it said the world had many chances to conserve water andto shift towards what it called a "blue economy". Fixing leakypipes could help - in developing nations, about 40 percent ofdomestic water is lost before it reaches households.

Nations such as Israel have limited water use, for instanceby shifting to less water-intensive crops or recycling. Olivesor dates need less water, for instance, than oranges.

The report said that annual spending on improving watersupplies and sanitation in developing nations should be raisedby about $11 billion a year. Every dollar spent would yield aneconomic return of $3 to $4, it estimated.

One billion people have no fresh water and 2 billion lackbasic sanitation. About 4,500 children die of water-relateddiseases every day - the equivalent of 10 jumbo jets falling outof the sky with no survivors, Chretien wrote.